Sloped cantilever beam electrode for a MEMS device

ABSTRACT

A method of tilting a micromirror includes providing a substrate, a sloped electrode outwardly from the substrate, and a sloped electrode positioning system outwardly from the substrate. The method also includes applying, by the sloped electrode positioning system, forces sufficient to position the sloped electrode in an orientation that slopes away from the substrate.

TECHNICAL FIELD

This invention relates in general to microelectromechanical systems(MEMS) and, in particular, to a sloped cantilever beam electrode for adigital micromirror device (DMD).

BACKGROUND

Microelectromechanical systems (MEMS) often comprise electrostaticfields in their operation. Digital micromirror devices (DMD) are aparticular MEMS device capable of being used in optical communicationand/or projection display systems. DMDs involve an array of micromirrorsthat selectively communicate at least a portion of an optical signal orlight beam. DMDs selectively communicate an optical signal or light beamby pivoting between active “on” and “off” states. To permit themicromirrors to pivot, each micromirror is attached to a hinge that issuspended between one or more support posts.

OVERVIEW OF EXAMPLE EMBODIMENTS

In one embodiment, an apparatus for use with a digital micromirrordevice (DMD) includes a substrate, a micromirror, and a pair ofelectrode systems each having an electrode positioning system. Themicromirror is disposed outwardly from the substrate and capable ofpivoting about a pivot point. The electrode systems are disposedinwardly from the micromirror on opposite sides of the pivot point. Eachelectrode system is operable to apply electrostatic forces on themicromirror in response to receiving a voltage. In addition, eachelectrode system has a sloped portion sloping away from the substrate.Each electrode positioning system is operable to position the slopedportion in an orientation sloping away from the substrate.

In a method embodiment, a method of tilting a micromirror includesproviding a substrate, a sloped electrode outwardly from the substrate,and a sloped electrode positioning system outwardly from the substrate.The method also includes applying, by the sloped electrode positioningsystem, forces sufficient to position the sloped electrode in anorientation that slopes away from the substrate.

Depending on the specific features implemented, particular embodimentsof the present invention may exhibit some, none, or all of the followingtechnical advantages. Various embodiments may be capable of enhancingthe electrostatic coupling between conductive layers, with benefits thatinclude, but are not limited to, balancing the electrostatic fieldsacross conductive layers, increasing the transitioning speed of eachmicromirror, and increasing the magnitude of the electrostatic fieldsbetween conductive layers. Some embodiments may be capable of enablingan increased micromirror thickness without compromising reliability.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, description and claims. Moreover,while specific advantages have been enumerated, various embodiments mayinclude all, some or none of the enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and forfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of one embodiment of a portion of a displaysystem;

FIG. 2A is perspective view of one embodiment of a portion of a digitalmicromirror device; and

FIGS. 2B through 2E are cross-sectional views illustrating examplemethods of forming a portion of a digital micromirror device.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Particular examples and dimensions specified throughout this documentare intended for example purposes only, and are not intended to limitthe scope of the present disclosure. In particular, this document is notintended to be limited to a particular microelectromechanical system(MEMS) device in a spatial light modulator application, such as, adigital micromirror device. Moreover, the illustrations in the FIGURESare not necessarily drawn to scale.

FIG. 1 is a block diagram of one embodiment of a portion of a displaysystem 10 that may be used with other embodiments of the invention. Thedisplay system 10 of FIG. 1 includes a light source module 12 capable ofgenerating illumination light beams 14. Light beams 14 are directed fromlight source module 12 to a modulator 16. Modulator 16 may comprise anydevice capable of selectively communicating at least some of thereceived light beams along a projection light path 18. In variousembodiments, modulator 16 may comprise a spatial light modulator, suchas, for example, a liquid crystal display, a light emitting diodemodulator, or a liquid crystal on silicon display. In the illustratedembodiment, however, modulator 16 comprises a digital micromirror device(DMD).

As will be described in more detail below, a DMD is amicroelectromechanical device comprising an array of hundreds ofthousands of tilting digital micromirrors. In a flat or neutral state,each micromirror may be substantially parallel to projection lens 24.From the flat state, the micromirrors may be tilted, for example, to apositive or negative angle corresponding to an “on” state and an “off”state. In particular embodiments, the micromirrors may tilt, forexample, from +12 degrees to a −12 degrees. Although particularembodiments, may have micromirrors that tilt from +12 degrees to a −12degrees, any other appropriate tilt angle may be used without departingfrom the scope of the present disclosure. To permit the micromirrors totilt, each micromirror attaches to one or more hinges mounted on supportposts, and spaced by means of an air gap over underlying controlcircuitry. The control circuitry provides the desired voltages to therespective layers, based at least in part on image data 20 received froma control module 22. In various embodiments, modulator 16 is capable ofgenerating various levels or shades for each color received.

Electrostatic forces cause each micromirror to selectively tilt.Incident illumination light on the micromirror array is reflected by the“on” micromirrors along projection path 18 for receipt by projectionlens 24. Additionally, illumination light beams 14 are reflected by the“off” micromirrors and directed on off-state light path 26 toward lightabsorber 28. The pattern of “on” versus “off” mirrors (e.g., light anddark mirrors) forms an image that is projected by projection lens 24. Asused in this document, the terms “micromirrors” and “pixels” are usedinter-changeably.

Light source module 12 includes one or more lamps or other light sourcescapable of generating and focusing an illumination light beam. Althoughdisplay system 10 is described and illustrated as including a singlelight source module 12, it is generally recognized that display system10 may include any suitable number of light sources modules appropriatefor generating light beams for transmission to modulator 16.

As discussed above, display system 10 includes a control module 22 thatreceives and relays image data 20 to modulator 16 to effect the tiltingof micromirrors in modulator 16. Specifically, control module 22 mayrelay image data 20 that identifies the appropriate tilt of themicromirrors of modulator 16. For example, control module 22 may sendimage data 20 to modulator 16 that indicates that specific micromirrorsof modulator 16 should be positioned in the “on” state. Accordingly, themicromirrors may be positioned at a tilt angle on the order ofapproximately +12 degrees, as measured from projection path 18.Alternatively, control module 22 may send image data 20 to modulator 16that indicates specific micromirrors should be positioned in the “off”state. As such, the micromirrors may be positioned at a tilt angle onthe order of approximately −12 degrees, as measured from projection path18.

FIG. 2A is a perspective view of one embodiment of a portion of adigital micromirror (DMD) device 200. As discussed above with regard tomodulator 16 of FIG. 1, DMD 200 may include an array of hundreds ofthousands of tilting micromirrors (e.g., micromirror 202). Eachmicromirror 202 is generally a portion of a pixel element 226 fabricatedmonolithically over a complementary metal-oxide semiconductor (“CMOS”)substrate 220. In particular embodiments, the CMOS substrate 220includes component parts of control circuitry operable to manipulatemicromirror 202. For example, the CMOS substrate 220 may include an SRAMcell or other similar structure for performing the operations of eachmicromirror 202. Each pixel element 226 may generally include a mirrorportion, a hinge portion, and an address portion.

The mirror portion of each pixel element 226 in the illustratedembodiment uses a reflective material such as aluminum or other materialto reflect incident light to produce an image through projection lens24. In some embodiments, the reflective material may be a micromirror202. In particular embodiments, the micromirror 202 may be approximately13.7 microns in size and have approximately a one micron gap betweenadjacent micromirrors. The described dimensions, however, are merely oneexample configuration of micromirrors 202. It is generally recognizedthat, in other embodiments, each micromirror 202 may be smaller orlarger than the above described example. For example, in particularembodiments, each micromirror may be less than thirteen microns in size.In other embodiments, each micromirror may be approximately seventeenmicrons in size.

The hinge portion of each pixel element 226, in the illustratedembodiment, includes one or more hinges 204 that are supported by hingeposts or hinge vias 214. Each hinge 204 may be made of aluminum,titanium, tungsten, aluminum alloys, such as AlTiO, or other materialsuitable for supporting and manipulating micromirrors 202. In operation,the one or more hinges 204 may be used to tilt each micromirror 202 suchthat the micromirrors 202 may be alternated between an active “on” stateor an active “off” state to selectively communicate at least a portionof an optical signal or light beam. For example, and as described abovewith regard to FIG. 1, hinges 204 may operate to tilt micromirrors 202from a plus twelve degrees to a minus twelve degrees to alternate themicromirrors 202 between the active “on” state condition and the active“off” state condition, respectively.

The micromirrors 202 are generally supported above the hinge 204 by amirror via 224. In the illustrated embodiment, the range of motion givento micromirrors 202 may be limited by spring-tip pairs 206 a and 206 bwithin the hinge layer. Thus, micromirrors 202 may be tilted in thepositive or negative direction until the micromirror 202 contacts andcompresses spring-tip pairs 206 a or 206 b respectively. Although thisexample includes spring-tip pairs 206 a, 206 b for limiting the motionof micromirrors 202 to a desired range, other embodiments may utilizeother means. For example, it is generally recognized that micromirrors202 may tilt in the positive or negative direction until micromirror 202contacts a spring-ring or until a beam or yoke coupled to the hingecontacts landing pads.

For conventional DMDS, the surfaces of the address portion of each pixelelement are typically disposed in planes that are parallel to thesubstrate or to the micromirror when in its neutral position. Duringoperation of such conventional DMDs, as the micromirror approaches itslanding position, portions of the micromirror are minimally spaced fromthe address portions, causing localized peak electrostatic fields. Theselocalized fields may cause undesirable micromirror dynamics, includingover-rotation and vertical hinge oscillation, which could causedestructive shorting between conductive layers of a pixel element. Inaddition, the electrostatic coupling within such conventional DMDs istypically limited to the overlap of each micromirror to two conductiveplanes.

Unlike conventional DMDs, the address portion of pixel element 226, inthe illustrated embodiment, includes a pair of sloped electrodes 210 inaddition to a pair of first electrodes 208 and a pair of secondelectrodes 212. In the illustrated embodiment, each sloped electrode 210comprises a cantilever beam coupled to its respective address electrodes212 and 208. Although the sloped profile of sloping surface 210 issubstantially linear or planar, other embodiments may alternatively havea sloped profile that is curved or bent without departing from the scopeof the present disclosure.

In operation, each sloped electrode 210 a and 210 b operates toredistribute the electrostatic fields along the length (1) of themicromirror 202 by balancing out localized field effects. In addition,each sloped electrode 210 a and 210 b introduces an additional plane ofelectrostatic coupling or attraction between the mirror portion andaddress portion of pixel element 226. That is, the surface of slopedelectrode 210 b, for example, may be disposed along a plane thatintersects the planes associated with the surfaces of electrodes 208 band 212 b. This additional plane of electrostatic coupling increases theconductive area associated with addressing micromirror 202 whileeffectively reducing the gap distance between the mirror portion andaddress portion of pixel element 226. The increased conductive area andreduced gap distance to micromirror 202 associated with each slopedelectrode 210 can advantageously form an enhanced electrostatic fieldcoupling within DMD 200.

In various embodiments, the enhanced electrostatic field couplingassociated with each sloped electrode 210 can operate to increase thecross-over transition speed of micromirror 202. The phrase “cross-overtransition speed” refers to the speed at which micromirror 202transitions between its “on” state and “off” state. In addition, theenhanced electrostatic fields associated with each sloped electrode 210more efficiently latches micromirror 202 in its active state, resultingin enhanced reliability.

The creation of electrostatic fields within each pixel element 226 maybe effected through any of a variety of means. For example, portions ofthe pixel element 226 may receive a bias voltage that at least partiallycontributes to the creation of the electrostatic forces (e.g., a voltagedifferential) between the address portions, which includes addresselectrodes 208, 210 and 212, and the micromirrors 202. That is, a biasvoltage may be applied to conductive conduit 216 that propagates throughhinge vias 214, along hinge 204 and through mirror via 224 to eachmicromirror 202. In particular embodiments, the latching bias voltagecomprises a steady-state voltage. That is, the bias voltage applied toconductive conduit 216 remains substantially constant while micromirror202 is in an “on-state” or “off-state” position. In this example, thelatching bias voltage comprises approximately twenty-six volts. Althoughthis example uses a bias voltage of twenty-six volts, other latchingbias voltages may be used without departing from the scope of thepresent disclosure.

As described above, CMOS substrate 220 comprises control circuitryassociated with DMD 200. The control circuitry may comprise anyhardware, software, firmware, or combination thereof capable of at leastpartially contributing to the creation of the electrostatic forcesbetween the address portions (e.g., address electrodes 208, 210, and212) and the micromirrors 202. The control circuitry associated withCMOS substrate 220 functions to selectively transition micromirrors 202between “on” and “off” states based at least in part on data receivedfrom a controller or processor (shown in FIG. 1 as reference numeral22). The control circuitry associated with CMOS substrate 220transitions micromirrors 202 between “on” and “off” states byselectively applying an address or control voltage to at least one ofthe address electrodes 212 a, 212 b electrically connected to respectiveelectrodes 208 and 210 associated with a particular micromirror 202. Inparticular embodiments, the control voltage is on the order ofapproximately three volts. Accordingly, to transition micromirror 202 tothe active “on” state condition, the control circuitry removes thecontrol voltage from electrode structure 212 a (reducing, for example,electrode 212 a from three volts to zero volts) and applies the controlvoltage to electrode structure 212 b (increasing, for example, electrode212 b from zero volts to three volts) before the micromirror receivesreset voltages. The combination of the electrostatic forces mayselectively create a torque force that transitions the micromirror.Although a control voltage of three volts is described above, a controlvoltage of three volts is merely one example of a control voltage thatmay be selectively applied to electrodes 212 a, 212 b. It is generallyrecognized that other control voltages may be used without departingfrom the scope of the present disclosure.

In the illustrated embodiment, address electrodes 208 a and 208 b areformed in the hinge layer and address electrodes 212 and the conductiveconduit 216 are formed within an inner conductive layer (also referredto sometimes as a Metal 3 or M3 layer). The inner conductive layer isdisposed outwardly from an oxide layer 222, which operates as aninsulator. For example, the oxide layer 222 may at least partiallyinsulate CMOS substrate 220 from address pads 212 a, 212 b andconductive conduit 216. As another example, the oxide layer 222 mayadditionally or alternatively operate to at least partially insulate theaddress electrodes 212 a, 212 b from the conductive conduit 216.

Address electrodes 212 and conductive conduit 216 may comprise, forexample, aluminum, an aluminum alloy or other conductive material. Whereaddress electrodes 212 and conductive conduit 216 comprise an aluminumalloy, the aluminum alloy may comprise, for example, aluminum, silicon,polysilicon, tungsten, nitride, and/or a combination of these or otherconductive materials. In this example, address electrodes 212 andconductive conduit 216 comprise silicon-based aluminum that has lightabsorbing and/or anti-reflective properties. In other embodiments,address electrodes 212 and conductive conduit 216 may include adielectric material with anti-reflective properties disposed outwardlyfrom the silicon-based aluminum layer.

By combining the DMD 200 with a suitable light source and projectionoptics (described above with regard to FIG. 1), the micromirror 202 mayreflect incident light either into or out of the pupil of the projectionlens 24. Thus, the “on” state of micromirror 202 appears bright and the“off” state of micromirror 202 appears dark. Gray scale may be achievedby binary pulse width modulation of the incident light. In someembodiments, color may be achieved by color filters, either stationaryor rotating, in combination with one, two, or three DMDs 200. Otherembodiments may achieve color by other means, such as, for example,colored light emitting diodes (LEDs).

In some implementations, keeping a sloped electrode 210 in a slopedposition may be difficult because of the large electrostatic attractionbetween the micromirror 202 and the sloped electrode during operation.This large electrostatic attraction may cause micromirror 202 and slopedelectrode 210 to collide. Contact between the sloped electrode 210 andthe micromirror 202 may cause a shorting event, or if separated by adielectric, may cause contact adhesion or disrupt the system dynamics ina way that inhibits reliability. Examples ways to address these issuesare described with respect to FIGS. 2B through 2E.

FIG. 2B is a cross sectional view illustrating one example of a methodof forming a portion of a digital micromirror device (DMD) 200. In thisparticular embodiment, each sloped electrode 210 is coupled to itsrespective address electrode 208. In addition, this particularembodiment comprises forming electrodes 228 a and an electrode 228 binwardly from each respective sloped electrode 210 a and 210 b.Electrodes 228 a and 228 b may be electrically interconnected to controlcircuitry (not explicitly shown) capable of at least partially creatingelectrostatic fields between each sloped electrode 210 and itsrespective electrode 228. A sufficient electrostatic field will causesloped electrodes 210 to slope inward from a less sloped position (asshown in FIG. 2C) to the position sloping away from micromirror 202shown in FIG. 23. For example, control circuitry may apply 30 volts toelectrodes to 228 while grounding micromirror 202 and at least one ofthe address electrode structures (e.g., 208 a, 210 a, and 212 a). Thus,because of the larger electrostatic field strength between slopedelectrode 210 a and electrode 228 a, sloped electrode 210 a slopesinward, or away from micromirror 202.

Some embodiments may use additional means to position sloped electrode210 while minimizing the risk of contact between micromirror 202 andsloped electrodes 210. For example, prior to tilting micromirrors 202,sloped electrodes 210 may be preconditioned to slope inward byapplication of electrostatic fields between electrode 228 and slopedelectrodes 210 over an extended period of time and at elevatedtemperatures. As another example, a dielectric layer (not explicitlyshown) disposed outwardly from sloped electrodes 210 may minimize therisk of shorting between sloped electrodes 210 and micromirrors 202. Insome embodiments, sloped electrode 210 may adhere or “micro-weld” toinwardly disposed electrodes, thus forming a permanently sloped profile.For example, some embodiments may form a dielectric layer (shown in FIG.2D as reference numeral 230) disposed outwardly from electrodes 208, 212and 228 that provides a contact surface to which sloped electrodes 210may adhere (as shown by reference 232 in FIG. 2D). In such embodiments,the voltage differential between sloped electrodes 210 and electrodes228 may permanently “micro-weld” sloped electrode 210 in a slopedposition. Likewise, contact between sloped electrode 210 and electrode212 may form a “micro-weld” (shown in FIG. 2D as reference numeral 234).As another example of contributing to the inward slope of slopedelectrode 210, in some embodiments, the material stresses associatedwith the formation of each electrode 210 may respond to elevatedtemperatures by deforming or curling the cantilever beam portioninwardly, towards its respective address electrode 212.

FIG. 2E is a cross sectional view illustrating one example of a methodof forming a portion of a digital micromirror device (DMD) 200. In thisparticular embodiment, each sloped electrode 210 is formed initiallywith a fixed end 209 that is coupled to its respective address electrode212 and a free end 211 that is free to pivot toward micromirror 202.During operation, as micromirror 202 tilts toward electrode 210, theincreasing electrostatic fields due to the proximity of micromirror 202with electrode 210 will cause the free end 211 of the respectiveelectrode 210 to slope outward. Sloped electrode 210 may be designed andpositioned to contact address electrode 208, thereby ensuring slopedelectrode 210 does not short to the tilting micromirror 202. Thus, inthis embodiment, sloped electrode 210 is positioned in a sloped positionwhen micromirror 202 tilts toward sloped electrode 210 and may be lesssloped when micromirror 202 is tilted away from sloped electrode 210.Although the sloped profile of sloping surface 210 is substantiallylinear or planar, other embodiments may alternatively have a slopedprofile that is curved or bent without departing from the scope of thepresent disclosure.

Some embodiments may use additional means to position sloped electrode210 while minimizing the risk of contact between micromirror 202 andsloped electrodes 210. For example, in some embodiments, contact betweensloped electrodes 210 and electrodes 208 may form a “micro-weld” thatpermanently positions sloped electrode 210 in a sloped position. Inaddition, in some embodiments, the material stresses associated with theformation of each electrode 210 may respond to elevated temperatures bydeforming or curling the cantilever beam portion outwardly, toward itsrespective address electrode 208.

Although the present invention has been described in severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfalling within the spirit and scope of the appended claims.

1. A digital micromirror device comprising: a substrate; a micromirrordisposed outwardly from the substrate and capable of pivoting about apivot point; a pair of electrode systems disposed inwardly from themicromirror and on opposite sides of the pivot point, each electrodesystem operable to apply electrostatic forces on the micromirror inresponse to receiving a voltage, and each electrode system having asloped portion sloping away from the substrate; and for each electrodesystem, an electrode positioning system operable to position the slopedportion in an orientation sloping away from the substrate.
 2. Thedigital micromirror device of claim 1, wherein the electrode positioningsystem comprises at least one electrode operable to apply electrostaticforces on the sloped electrode that at least partially contribute toorienting the sloped portion in the sloped position.
 3. A lightprocessing system, comprising: a light source; a substrate; a digitalmicromirror device disposed outwardly from the substrate and comprisinga plurality of pixel elements each comprising a micromirror capable ofpivoting to selectively reflect light from the light source; for eachmicromirror, a pair of electrode systems disposed inwardly from themicromirror and on opposite sides of a pivot point of the micromirror,each electrode system operable to apply electrostatic forces on themicromirror, and having a sloped electrode; and for each electrodesystem, a sloped electrode positioning system operable to position thesloped electrode in an orientation sloping away from the substrate. 4.The light processing system of claim 3, wherein the sloped electrodepositioning system is operable to produce electrostatic fields generatedbetween the sloped electrode and a different electrode disposed inwardlyfrom the sloped electrode that at least partially contribute to theposition of the sloped electrode in an orientation sloping away from thesubstrate.
 5. The light processing system of claim 3, wherein the slopedelectrode positioning system is operable to produce electrostatic fieldsbetween the sloped electrode and the micromirror that at least partiallycontribute to the position of the sloped electrode in an orientationsloping away from the substrate.
 6. The light processing system of claim5, and further comprising a superstructure element disposed outwardlyfrom the sloped electrode positioned to prevent the sloped electrodefrom pivoting into the micromirror.
 7. The light processing system ofclaim 6, wherein the superstructure element comprises an electrodeoperable to generate electrostatic fields between the superstructureelement and the micromirror.
 8. The light processing system of claim 3,wherein the sloped electrode positioning system is operable to pivot afree end of the sloped electrode until it permanently attaches to adifferent electrode.
 9. The light processing system of claim 8, whereinthe sloped electrode positioning system is operable to produce a voltagedifferential between the sloped electrode and the different electrodethat at least partially contributes to the permanent attachment of thesloped electrode to the different electrode.
 10. The light processingsystem of claim 8, wherein the different electrode comprises adielectric layer disposed outwardly from the different electrode. 11.The light processing system of claim 3, wherein the sloped electrodepositioning system is operable to produce material stresses associatedwith the formation of each sloped electrode that at least partiallycontribute to the position of the sloped electrode in an orientationsloping away from the substrate.
 12. The light processing system ofclaim 3, wherein the sloped electrode comprises a plurality of layers,at least one layer comprising material having insulator properties.